Cribrum

A thin, domed, porous plate of silica covering the internal opening of an areola, found in genera such as Thalassiosira and Stephanodiscus. A cribrum is a type of
velum.
Visible only with the SEM. Plural form is cribra.

A similar structure with finer pores, found in many raphid diatoms, is the
hymen.